The present invention relates to polyolefin films and biaxially oriented films with improved properties useable as shrink films. In particular, the present invention relates to shrink film for use in packaging lines, although the invention may also be used in other shrink film applications.
Polyolefins, polyvinyl chlorides, ionomers, polyesters, polystyrenes, and polyvinylidene chlorides have all been used in the production of shrink film. The shrinkable polyolefins currently on the market include both cross-linked and non cross-linked oriented polyethylene, oriented polypropylene, and oriented ethylene-propylene copolymers.
A shrink film's distinguishing characteristic is its ability upon exposure to heat to either shrink or, if restrained, to create shrink tension within the film. This ability is activated by the packager when the wrapped product is passed through a hot air or hot water shrink tunnel. This process causes the film to shrink around the product producing a tight, transparent wrapping that conforms to the contour of the product and which is aesthetically pleasing while providing the useful functions required of packaging materials such as protection of the product from loss of components, pilferage, damage due to handling and shipment, dirt and contamination.
Typical items wrapped in polyolefin shrink films are toys, games, sporting goods, stationery, greeting cards, hardware and household products, office supplies and forms, foods, phonograph records, and industrial parts.
The manufacture of shrink films requires sophisticated equipment including extrusion lines with “biaxial orientation” capability. Biaxial orientation can be accomplished with “tenter frame” stretching or “double bubble” blown film extrusion processes. The biaxial orientation causes the material to be stretched in the cross or transverse direction and in the longitudinal or machine direction. The films are usually heated to their orientation temperature range that varies with the different polymers but is usually above room temperature and below the polymer's melting temperature. After being stretched, the film is rapidly cooled to quench it thus freezing the molecules of the film in their oriented state. Upon heating, the orientation stresses are released and the film will begin to shrink back to its original unoriented dimension.
The polyolefin family of shrink films provide a wide range of physical and performance characteristics such as shrink force (the amount of force that a film exerts per unit area of its cross-section during shrinkage), the degree of free shrink (the reduction in surface area a material undergoes when unrestrained), tensile strength (the highest force that can be applied to a unit area of film before it begins to break), sealability, shrink temperature curve (the relationship of shrink to temperature), tear initiation and resistance (the force at which a film will begin to tear and continue to tear), optics (gloss, haze and transparency of material), and dimensional stability (the ability of the film to retain its original dimensions under all types of storage conditions).
In addition to the above, high speed automatic wrapping lines require that the shrink film have high film-film slip (low coefficient of friction), consistently strong static seals and higher stiffness compared to other shrink films. The strong static seals allow for fast sealing of the packages. Higher stiffness supports high film and packaging line speeds. Stiffness is measured as Young's modulus or “modulus” per ASTM test D 882.
High slip allows packages to freely pass one another on conveyors without sticking or clinging. Slip may be determined at room temperature or at elevated temperatures. Slip at room temperature is representative of conditions packaged goods encounter during transportation between facilities and during loading and off-loading of packages from transport vehicles.
Packages having high slip at room temperature may still have poor hot slip characteristics. Hot slip is important for operating packaging lines at high speeds. One determinant in line speed is the amount of time required to cool the packages downstream of the shrink oven or chamber. Packages must be cooled to a point where they no longer cling to other packages. For this reason, packages with superior hot slip require less cooling time and therefore allow faster packaging line speeds.
One example of a commercially successful polyethylenic based shrink film requires a silicone anti-block agent to provide sufficient hot slip. In this product, approximately 0.4% of the outer layers comprises spherical silicone (10% concentration in a LLDPE based master batch blend).
Propylene copolymers and silicon oils have been used to enhance film hot slip characteristics. For example, U.S. Pat. No. 5,298,302 to Boice uses skin layers comprising a propylene polymer or copolymer with butene or polybutylene. Such films often have limited tear propagation resistance. U.S. Pat. No. 6,291,063 to Shah et al. uses silicon oil in the form of organosiloxane in the outer film layers to improve hot slip.
The use of silicon compounds is undesirable due to public association with silicone toxicity. In addition, silicone migration or volatilization is problematic for silicone-sensitive end uses, such as in the microelectronics and automotive coatings industries. Further the use of silicones can cause printing problems such as poor adhesion or non-wetting of the ink.
Ticona manufactures cyclic-olefin copolymers (COC) and has found them helpful in cast films comprising low density polyethylene. Such films are often used as stretch films and are not the biaxially oriented shrink films of the present invention. Further, recommendations from Ticona indicate that COCs having a glass transition temperature within 100-104 degrees Celsius of the film extruder temperature must be selected. Surprisingly the inventors have been able to achieve superior hot slip in LLDPE and polypropylene terpolymer shrink films by employing COCs with glass transition temperatures 152 degrees Celsius below the extruder processing temperature.
Accordingly, it is a general object of the present invention to provide shrinkable films having improved properties for high speed packaging lines with superior hot slip characteristics and essentially silicone free surfaces.